1. Field of the Invention
The present invention relates to a constant voltage circuit, and in particular to a constant voltage output device employing field-effect transistors.
2. Description of the Prior Art
FIG. 4 shows a conventional constant voltage output device employing bipolar transistors. The transistors Q17 and Q18 form a current mirror circuit having a resistor R6 connected to the emitter of the transistor Q18. Let the base-emitter voltage of the transistor Q17 be VF.sub.17 and that of the transistor Q18 be VF.sub.18, and assume that the capacity of the transistor Q18 is 10 times that of the transistor Q17. The resistance of the resistor R6 is so determined that, in the steady state, the current I.sub.6 flowing through the transistor Q17 and the current I.sub.7 flowing through the transistor Q18 are equal. Then, the voltage across the resistor R6 is expressed as ##EQU1## where V.sub.T =kT/q (with k representing Boltzmann's constant, T representing absolute temperature, and q representing the charge of an electron), and I.sub.S represents the saturation current of the transistor Q17. Hence, the current I.sub.7 ' that flows through the resistor R6 can be expressed approximately as ##EQU2##
If it is assumed that the capacities of the transistors Q16 and Q19 are equal, the current I.sub.8 flowing through the resistor R7 and the diode D5 is equal to the current I.sub.7. Let the forward voltage of the diode D5 be VF.sub.5. Then, the output voltage Vout is expressed as ##EQU3##
The output voltage Vout depends on the ratio between the resistances of the resistors R6 and R7 and on the ratio between the capacities of the transistors Q17 and Q18. Note that, when the circuit is started up, the transistors Q17 and Q18 are turned on by the current supplied from a starting circuit 16.
However, quite inconveniently, it has been impossible to construct a constant voltage output device that operates in the same way as the above-described conventional constant voltage output device by the use of field-effect transistors instead of bipolar transistors. In digital IC (integrated circuit) devices composed of field-effect transistors, using bipolar transistors solely to compose their constant voltage circuit sections leads to extra production cost.